1. Field of the Invention
The present invention relates to a recording ink composition suitable for an ink-jet printer, aqueous writing utensils, various kinds of recorders and pen plotters, and more particularly to a recording ink composition capable of producing color images on a sheet of plain paper by an ink-jet printing method. The present invention also relates to a method of recording images, using the above-mentioned recording ink composition.
2. Discussion of Background
Ink-jet printers have been widely utilized in recent years because of the advantages of low noise and low running cost, and color ink-jet printers capable of producing color images on a sheet of plain paper have also been placed on the market.
With respect to such ink-jet printers, however, it is required to satisfy all the requirements, such as excellent color reproduction of color image, high water- and light-resistances of the obtained image, sufficient drying characteristics of the image, high image quality without blurring, and the reliability of ink-ejection from the printer.
When the color images are produced, in particular, by using the color ink-jet printer, image blurring easily takes place in two-color-superimposed color image portions of red color, green or blue color and also in the boundaries thereof, even though image clearness is sufficient in a single-colored image portion, using a yellow, magenta or cyan color ink composition of a single color.
Particularly, when an ink deposited on a sheet of paper is dried without any specific image-fixing unit, the drying characteristics of the ink are improved by increasing the penetrating property of the ink through the paper as described in Japanese Laid-Open Patent Application 55-29546. However, because of the improved penetrating property of the ink through paper, blurred images are apt to be formed and the sharpness of the obtained images is lowered depending on the kind of recording paper to be employed.
In Japanese Patent Publication 60-23793, it is described that a dialkylsulfosuccinate is contained in the ink composition thereof as a surfactant, whereby the drying characteristics of images obtained by ink-jet recording can be improved and the reduction of the sharpness of the obtained images is minimized. However, when such an ink composition is used in ink-jet printing, the diameters of picture elements of the obtained image considerably vary depending on the kind of image receiving sheet to be employed. As a result, the image density of the printed images is considerably decreased, and the sharpness of the printed images is lowered. In addition, this kind of surfactant is easily decomposed if the pH of the ink composition is on an alkaline side, so that the effect of the surfactant is lost during the storage of the ink composition and precipitates are formed in the ink composition. As a result, the nozzles of an ink-jet printer may be easily clogged with the precipitates in the ink composition, which will lower the reliability of ink-ejection performance.
In Japanese Laid-Open Patent Application 56-57862, it is disclosed that the drying characteristics of the images recorded by ink-jet printing can be improved by using an ink composition in which a strongly basic material is contained. In the case where images are formed, using this ink composition, on a so-called acidic paper which has been sized using rosin, the drying characteristics of the images can be improved. However, in the case of a paper in which an alkyl ketene dimer or an alkenyl sulfosuccinic acid is used as a sizing agent, no improvement is observed in the drying characteristics of images formed on the paper. Even in the case of the acidic paper, no improvement is observed in the drying characteristics of the two-color-superimposed image portion on the acidic paper.
In Japanese Laid-Open Patent Application 1-203483, there is proposed a recording ink composition comprising a polyhydric alcohol derivative and pectin, in which pectin serves as a thickening agent for preventing the spreading or the blurring of recorded images. However, pectin is a nonionic material having a hydroxyl group as a hydrophilic group, so that it has the shortcoming that ink ejection reliability is low when the printing operation is resumed after intermission.
Dyes for the ink composition for use in ink-jet printing are being improved in order to obtain satisfactory color reproduction performance and excellent light resistance and water resistance of color images formed on a sheet of plain paper at the same time. However, it is still difficult to obtain a dye for an ink composition which is capable of producing color images with improved light and water resistances, while retaining high reliability of ink-ejection performance.
It is conventionally proposed to use various pigments for the ink composition in order to obtain satisfactory water resistance and light resistance of the recorded ink image. However, such pigments have a problem with respect to color tone, and therefore are now used only in an ink composition for a large-sized plotter which is required to produce images with extremely high light resistance. When such a pigment is used in an ink composition for an ink-jet printer which is designed to produce images with high resolution, there occurs a problem that the nozzle of the printer is easily clogged with the ink composition, so that the reliability of the ink ejection performance is extremely lowered.
In Japanese Patent Publication 60-45669 and Japanese Laid-Open Patent Application 6-100812, it is proposed to use a pigment and a dye in combination in order to improve the color tone, the color reproduction performance, the light resistance and water resistance of the printed images, and the reliability of the ink ejection performance.
The ink composition disclosed in Japanese Patent Publication 60-45669 has the problems that the light resistance of the employed dye is insufficient for use in practice, the penetrating performance of the ink composition through plain paper is not sufficient and that the color development performance thereof is not satisfactory.
The ink composition disclosed in Japanese Laid-open Patent Application 6-100812 has sufficiently good light resistance in comparison with the ink composition disclosed in Japanese Patent Publication 60-45669, but has the problems that the color development performance is poorer than that of the ink composition disclosed in Japanese Patent Publication 60-45669, and that two-color superimposed image portions tend to become dull, in particular, with respect to the development of a blue color.
In Japanese Laid-Open Patent Applications 60-38481 and 60-38482, there are disclosed recording ink compositions in which a polymeric dye having a sulfonic acid group which is in the form of a salt in combination with a basic dye. These recording ink compositions have the shortcomings that the light resistance thereof is insufficient for use in practice and that the nozzles of the ink-jet printer are easily clogged with the recording ink compositions, and lack in the reliability of ink ejection performance.
Thus, it is extremely difficult to obtain a recording ink composition with excellent color tone and color reproduction performance, which is capable of producing clear color images free from blurring and spreading, with high water resistance and light resistance, and is suitable for ink-jet printing with high reliability of ink-ejection performance.